This invention relates to an optical switch and an optical distributor. More particularly, the invention relates to an optical switch, which is placed between the transmitting and receiving sections of an optical signal, or in a transmission line, for switching optical signals, as well as to an optical distributor for distributing optical signals.
In the field of communications and information processing in recent years, an optical-signal network has to be required in order to increase the volume of information handled and raise system operating speed. In order to realize a highly sophisticated network, there is a need for an optical switch and optical distributor for connecting or switching a large number of signals.
To achieve this, research in optical switches has progressed and a variety of optical switches have been proposed. FIGS. 23A.about.23E are diagrams for describing reflection-type optical switches proposed by S. H. Song, et al (see S. H. Song, et. al., Optics Letters, Vol. 17, No. 18, 1992). One type of optical switch includes a glass substrate GPL composed of two layers, a polarizing separating film PBS coating the top surface of the glass. substrate GPL, a microprism MPZ formed by subjecting the surface of the glass substrate over the polarizing separating film PBS to micromachining, and a mirror MR formed on the other surface of the glass substrate (see FIGS. 23B and 23D). The reflection-type optical switch controls the state of polarization of incident light (e.g., transversely polarized light) at a polarization controller (not shown), inputs the incident light, whose state of polarization has been controlled, to the polarizing separating film PBS, transmits and reflects the incident light at the polarizing separating film PBS in dependence upon the state of polarization, and reflects the transmitted incident light twice at the inner side of the microprism MPZ, thereby changing the optical path.
For example, in FIG. 23B, transversely polarized light (a P wave) inputted at #1 is reflected by the mirror MR, after which the light passes through the polarizing separating film PBS and reaches the microprism MPZ, at which the light is reflected twice so as to be outputted from output optical path #1 (see FIG. 23D). In this case there is no change in the optical path. However, when the state of polarization of the incident light is controlled by the polarization controller (not shown) so that a change is made to longitudinal polarization (an S wave), the incident light is reflected by the polarizing separating film PBS and is outputted from optical path #4. Thus, the optical path is switched. Similarly, optical signals which have entered from #2.about.#4 also undergo a switch in optical path depending upon the state of polarization. Accordingly, a network illustrated on the left side of FIG. 23A is constructed by the optical switch of FIG. 23B.
FIG. 23C illustrates an optical switch for constructing another network. This type of optical switch has two small microprisms MPZ1, MPZ2 formed in the surface of the glass substrate GPL. Transversely polarized light (a P wave) inputted at #1 is reflected by the mirror MR, after which the light passes through the polarizing separating film PBS and reaches the microprism MPZ1, at which the light is reflected twice so as to be outputted from output optical path #1 (see FIG. 23E). In this case there is no change in the optical path. However, when the state of polarization of the incident light is controlled by the polarization controller so that a change is made to longitudinal polarization (an S wave), the incident light is reflected by the polarizing separating film PBS and is outputted from optical path #2. Thus, the optical path is switched. Similarly, optical signals which have entered from #2.about.#4 also undergo a switch in optical path depending upon the state of polarization. A network illustrated on the right side of FIG. 23A is constructed by the optical switch of FIG. 23C. Accordingly, when the switches shown in FIGS. 23B and 23C are optically coupled, the overall network depicted in FIG. 23A is constructed, and incident light which has entered from each of the input optical paths #1.about.#4 can be outputted from any one of the output optical paths #1.about.#4.
FIGS. 24A-E show a prior-art example of a transmission-type optical switch. This optical switch has been disclosed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 3-204621. FIG. 24A illustrates a four-input, four-output optical switch comprising liquid-crystal cells LC1.about.LC3, polarizing prisms P1, P1', P2, P2', GRIN lenses GR, optical fibers F and fiber collimator arrays CA each mounting a plurality of the GRIN lenses GR. An AC voltage is applied to each of the liquid-crystal cells LC1.about.LC3 via conductors. Turning the AC voltage on and off controls the polarization of the incident light so as to switch the optical path. As shown in FIGS. 24B and 24C, each of the liquid-crystal cells LC1.about.LC3 comprises transparent electrodes EL1.about.EL4 provided on each of its opposing sides, a rubbing film CF of polyimide or the like for aligning the liquid crystal, two glass substrates SU arranged to maintain a fixed spacing between them, and liquid crystal LI sealed in the gap between the glass substrates. The edges of the gap are sealed by a sealant AD such as epoxy resin in order to prevent outflow of the liquid crystal. The electrode patterns are arranged as shown in FIG. 24C in order that the respective optical paths may be switched independently of one another, and conductors WI1.about.WI4 for applying voltage to the electrodes are connected to the respective electrodes. Examples of the liquid crystal used are twisted nematic liquid crystal and ferroelectric liquid crystal. Twisted nematic liquid crystal does not experience rotation of polarization plane under application of a voltage but the plane of polarization does rotate by 90.degree. when voltage is removed. Ferroelectric liquid crystal can be switched between a state in which the polarization plane is rotated and a state in which it is not by applying pulses of different polarity (+, -) thereto. The liquid-crystal cell shown in FIG. 24A employs twisted nematic liquid crystal.
As shown in FIGS. 24D, 24E, each of the polarizing prisms P1, P1', P2, P2' has the shape of a parallelepiped with an apex angle of 45.degree.. A polarizing separating film PM and a total internal reflecting film M are arranged as shown. The polarizing separating film PM transmits horizontally polarized light (P waves) and reflects vertically polarized light (S waves). More specifically, S waves are reflected by the polarizing separating film PM, after which the P waves are reflected again at the end of the prism before exiting. The P waves pass through the polarizing separating film PM and then exit from the prism. The total internal reflecting film M reflects light without relation to the state of polarization, and light incident upon either surface of the film is reflected. The polarizing prisms P1, P1', P2, P2' and liquid-crystal cells LC1.about.LC3 are arranged alternatingly and combined in multiple stages, as shown in FIG. 24A, and the elements are secured by a bonding agent to construct the four-input, four-output optical switch.
FIGS. 25A, 25B are diagrams for describing the switching of the optical paths in the optical switch of FIG. 24. Here components identical with those shown in FIG. 24 are designated by like reference characters. First and third electrodes E1, E3 of liquid-crystal cell LC1 are turned off, second and third electrodes E2, E3 of liquid-crystal cell LC2 are turned on, and first and third electrodes E1, E3 of liquid-crystal cell LC3 are turned off (off electrodes are shown in black and on electrodes are indicated by hatching).
As shown in FIG. 25A, vertically polarized light (S waves) in the incident light from incoming line 01 is reflected by the polarizing separating film PM of the polarizing prism P1, after which the reflected light is reflected again at the end of the polarizing prism before passing through the liquid-crystal cell LC1. At this time the first electrode of the liquid-crystal cell LC1 is off and, hence, the incident light is subjected to control of its polarization and is changed to horizontally polarized light (P waves). The incident light composed of P waves passes through the polarizing separating film PM of the polarizing prism P2 and is reflected by the total internal reflecting film M before passing through the liquid-crystal cell LC2. Since the second electrode of the liquid-crystal cell LC2 is on, the incident light undergoes no change in polarization and reaches the total internal reflecting film M of the polarizing prism P2'. Here the light is reflected, passes through the polarizing separating film PM, is reflected again at the end of the polarizing prism P2' and then passes through the liquid-crystal cell LC3. Since the first electrode of the liquid-crystal cell LC3 is off, the incident light is changed to vertically polarized light (S waves). Incident light composed of S waves is reflected at the end of the polarizing prism P1', after which the light is reflected again by the polarizing separating film PM before exiting from outgoing line O1'.
Similarly, horizontally polarized light (P waves) in the incident light from incoming line O1 propagates through the switch and is outputted from outgoing line O1' in the manner shown in FIG. 25B. Accordingly, if the electrodes of each of the liquid-crystal cells are turned on and off in the manner set forth above, light incident from incoming line O1 is outputted from outgoing line O1' without any change in optical path. However, (1) if the first and third electrodes of liquid-crystal cell LC1 are turned off, the second and third electrodes of liquid-crystal cell LC2 are turned off and the first and third electrodes of liquid-crystal cell LC3 are turned on, the light incident from incoming line O1 is outputted from outgoing line 02'. (2) If the first and third electrodes of liquid-crystal cell LC1 are turned off, the second and third electrodes of liquid-crystal cell LC2 are turned on and the first and third electrodes of liquid-crystal cell LC3 are turned on, the light incident from incoming line O1 is outputted from outgoing line 03'. (3) If the first and third electrodes of liquid-crystal cell LC1 are turned off, the second and third electrodes of liquid-crystal cell LC2 are turned off and the second and fourth electrodes of liquid-crystal cell LC3 are turned off, the light incident from incoming line O1 is outputted from outgoing line O4'. In other words, light incident from incoming line O1 can be outputted on any outgoing line to change the optical path. Likewise, light incident from any other incoming line can be outputted on any outgoing line.
With the construction of the reflection-type optical switch of the prior art, it is required that the microprisms be fabricated so that their angles are extremely precise. In addition, it is required that light strike the positions of the microprisms accurately. As a consequence, it is difficult to fabricate the conventional reflection-type optical switch. Moreover, aligning the optic axes is laborious
Further, with the transmission-type optical switch of the prior art, the switch is simply reduced in size when designed for multiple-channel application. When the number of channels increases, therefore, it is required that a very large number of polarizing prisms be pasted together. Fabrication is a laborious task and there is a decline in fabrication precision.